1. Field of the Invention
The present invention relates to a method for detecting defects in metal structures and welds.
Metal structures and welds are invariably subjected to stress and fatigue conditions which give rise to the development of cracks or breaks internally or on the surface. The problem of such defects whether internal or external is even more acute in under-water or sub-sea structures which have to contend additionally with problems of corrosion.
Hitherto magnetic particle inspection has been used extensively to detect such defects especially in metal structures under water. This method relies on the fact that when a metal is magnetised, iron particles will be attracted to any change of flux of that field such as that caused by a defect whether internal or external. A cluster of these magnetic particles around a defect is termed an "indication" in the art. If a fluorescent dye is impregnated on the iron particles to produce the so-called "magnetic ink", then any particle clusters formed can be inspected by their fluorescence e.g. under UV light.
In the case of under-water structures a record of the "indication" is normally produced by a diver who photographs the "indication". This method, although useful, fails to produce a three-dimensional profile of the affected or defective area of a structure. Moreover, the diver has to be trained in the techniques involving knowledge of magnetic fields and flux, the art of applying the magnetic ink, visual appreciation of the nature and the extent of the damage to the affected area of the structure and finally ultraviolet photography of the surface treated with the normally fluorescent magnetic ink. In addition such UV photography has of necessity to be carried out under cover of darkness.
The absence of a three-dimensional profile of the affected area of the structure reduces the ability of the experts scrutinising the UV photograph to assess the damage accurately. Moreover, the need to carry out UV photography under darkness reduces considerably the flexiblity of the operation.
2. Description of Related Art
In an article by R. Purdy in the Journal "Non-Destructive Testing", November 1968, pp 363-369 a replica technique for recording magnetic particle indications is described. However, Purdy found that cold curing rubbers took too long to cure, e.g. 12 hours, and resorted to heat curable resins such as PVC. GB-A-934116 describes the use of PVC for a similar purpose and also employs a heat curing process. The heat curing process, however, was unsuccessful with fluorescent inks as the plasticiser component of the PVC paste absorbed the dye. The problem of slow cure or heat application is even more acute for under-water uses.